Threshold
thresh·old /ˈTHreSHˌ(h)ōld/
Noun 1. a limit or boundary beyond which a particular effect or process is initiated Every mystery solved brings us to the threshold of a greater one. - Rachel Carson (1907–1964) 2. a piece of timber or metal that lies beneath a door; an entrance Up to the threshold of the door there mounted a flight of twenty-seven broad stairs, hewn by some unknown art of the same black stone. The Two Towers (1954), by J.R.R. Tolkien (1892-1973)